1. Field of the Invention
This invention relates in general to power line carrier communication systems, and more specifically, to power line carrier communication systems which utilize a utility power distribution network for communication.
2. Description of the Prior Art
A power distribution network is polyphase in design and requires multi-conductors. The distribution primary is that portion of the distribution system between the distribution substation and the distribution transformer. Typically, the primary side of the distribution system is a three-phase network consisting of three or four conductors. The secondary side of the distribution system is that portion of the system between the primary feeders and the consumer's premises. The secondary side of the distribution system can be either single-phase or polyphase. In either case, the secondary side will typically consist of multi-conductor, two or three-phase service. The power distribution network is designed for the efficient transfer of power at fifty or sixty hertz. While this type of system works well for the distribution of power at power line frequencies, it presents severe problems when used as a communication system.
One problem associated with using the power distribution network as a communication system stems from the fact that the power distribution network is designed to transmit fifty or sixty hertz signals while the carrier communication signal is typically 5 kilohertz to 100 kilohertz. These high frequency communication signals experience extreme attenuation when impressed upon a power line conductor designed for lower frequency signals. Also, the power distribution network is an extremely noisy environment and suffers from large numbers of transients due to the switching on and off of user loads. The switching on and off of user loads may even change the characteristics of the power line conductor, thus effecting the rate of propagation of the carrier communication signal.
One prior art solution is to impress the carrier communication signal upon the phase conductor having the most favorable propagation characteristics. As the characteristics of the phase conductors change with the addition and removal of user loads, the phase conductor to which the carrier communication signal is impressed will also change. However, this presents bookkeeping problems in that each device which the user intends to communicate with must be switched to the proper phase conductor so as to receive the communication signal.
Another prior art solution is to impress the communication signal upon all of the three-phase conductors. Two different methods of impressing the communication signal on each of the three-phase conductors are disclosed in U.S. Pat. Nos. 4,065,763 and 4,188,619. By impressing the communication signal upon each of the three-phase conductors, the bookkeeping problem encountered in the prior art technique discussed above is eliminated.
In U.S. Pat. No. 4,065,763, an impedance matching transformer having multiple secondary taps is used for coupling the communication signal to each of the three-phase conductors. While such a scheme provides an inexpensive method of coupling the communication signal, it suffers from having to use the same magnetic material, i.e., the core of the impedance matching transformer, to couple all three communication signals. When coupling the communication signal to the power distribution network, any noise associated with the signal is coupled to each of the three-phase conductors. When removing the communication signals from the power distribution network, the communication signals will have slightly different phases due to the different propagation rates of each of the three-phase conductors. By using a single magnetic core to remove the communication signals from the power distribution network, the communication signals themselves will subtract because of the phase differences while the noise will add. Thus, the signal utilized by the receiving device will never be better than the best of the received signals, and will typically be worse.
U.S. Pat. No. 4,188,619 also discloses a method of impressing a communication signal upon each of the three-phase conductors. This method involves the use of a three-phase transformer or, in the alternative, three single-phase transformers. Although this method eliminates the burden placed upon the magnetic material, i.e., the cores of the transformers, the connection of the transformers presents substantially the same problem. That is, any noise associated with the communication signal will be coupled to each of the three-phase conductors. When removing the communication signal from each of the three-phase conductors, the communication signals will subtract while the noise associated with each signal will add. Thus, the signal utilized by the received device will never be better than the best of the received signals, and will typically be worse.
The present invention is for a three-phase power distribution network communication system wherein the signal utilized by the receiving device is typically better than, or equal to in a worst case analysis, the best of the received communication signals.